Testing and Troubleshooting 4-20 mA Control Loops Presented by Fluke and Transcat
- 1. Testing and Troubleshooting 4 mA to 20 mA Control Loops Process Troubleshooting Webinar Series
- 2. What will be covered in this session • What makes a 4 to 20 mA control loop “tick” • What things can go wrong with a 4 to 20 mA loop? • How do you troubleshoot? What tools are of most use? • Measuring 4 to 20 mA signals and interpreting the measurements – Multiple measurement methods • Testing PLC, DCS and indicator inputs with 4 to 20 mA sourcing tools – Multiple methods and sourcing approaches – Testing 1V to 5 V and 0 V to 10 V I/O ©2010 Fluke Corporation Troubleshooting 4 mA to 20 mA loops 2
- 3. What will be covered in this session • Measuring loop power supplies, troubleshooting suspect power supplies – Testing and isolating a transmitter with substitution power supply • Troubleshooting suspect transmitters – mA simulate used for substitution testing • Using a HART smart transmitter as a mA source – Using the Loop Test HART command to test a loop • Testing an I/P with 4 to 20 mA sourcing • Testing a control valve with a 4 to 20 mA source ©2010 Fluke Corporation Troubleshooting 4 mA to 20 mA loops 3
- 4. 24 V loop supply +– What makes a 4 to 20 mA control loop “tick” Indicators/Controllers: Interpret the 4 to 20 mA signal as the temperature or pressure being measured in the process. Often issue commands to a final control element such as a valve to regulate the process temperature or pressure to within acceptable limits Inputs to these devices is frequently a 1 V to 5 V input signal rather than 4 to 20 mA ZERO SPAN 2200 ºC 4 to 20 mA 2 Wire Transmitter Sensor input • Temperature • Pressure • Flow • Frequency • PH Readout/Controller DCS/PLC/Recorder 250 ohm input shunt 4 to 20 mA signal 24 V loop power supply: Provides power for the loop. Transmitters regulate the 4 to 20 mA signal in the circuit drawing power from this supply. Transmitters: Convert the measured temperature or pressure to a 4 to 20 mA signal. Typically are passive devices and draw their power from an external 24 V loop power supply.
- 5. 2200 ºC Example current loop ZERO SPAN 4 to 20 mA 2 Wire Transmitter Sensor input • Temperature • Pressure • Flow • Frequency • PH Readout/Controller DCS/PLC/Recorder 250 ohm input shunt 4 to 20 mA signal 24 V loop supply +– • 4 to 20 mA (dc) signal is proportional to sensor input or PV • Series circuit dictates the current at one location must be identical to other locations • Big advantage sending in sending mA signals over long distances compared to voltage or pressure signals
- 6. Temperature transmitters convert measured temperature (PV) 4 to 20 mA signals Measured PV: The PV or primary/process variable in this example is the temperature of the process fluid being measured by the temperature transmitter. Temperature transmitter: Typically sense the temperature of the process via either a thermocouple or RTD sensor and convert the measurement to a 4 to 20 mA signal. The transmitter in this example has an input span (range) of 0 °C to 300 °C. The relationship of the measured temperature and mA signal is shown is the table. Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter
- 7. Temperature transmitters convert measured temperature (PV) 4 to 20 mA signals Measured PV: The PV or primary/process variable in this example is the temperature being measured by the temperature transmitter. Temperature transmitter: Typically sense the temperature of the process via either a thermocouple or RTD sensor and convert the measurement to a 4 to 20 mA signal. The transmitter in this example has an input span (range) of 0 °C to 300 °C. The relationship of the measured temperature and mA signal is shown is the table. Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 %
- 8. Temperature Transmitters convert measured temperature (PV) 4 to 20 mA signals Indicator/Controller: In this example, the temperature indicator/controller is programmed to interpret a 4 mA signal as 0 °C and a 20 mA signal as 300 °C. Input/Output or I/O: Refers to the input/output of the control system or controller. In this example the input to the controller is the 4 to 20 mA signal. The output from the controller is the signal that controls the control valve. Flow Control Valve Final control element. Opens and closes based on commands from the controller to increase or decrease gas supplied to the burner as required to maintain temperature at the setpoint value. Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter
- 9. Pressure Transmitters convert measured pressure (PV) to 4 to 20 mA signals Measured PV: The PV or primary/process variable in this example is the pressure being measured by the pressure transmitter. Pressure Transmitter: Senses the pressure of the process directly and converts the measurement to a 4 to 20 mA signal. The transmitter in this example has an input span (range) of 0 psi to 100 psi. The relationship of the measured pressure and mA signal is shown is the table. Indicators/Controller: In this example, the pressure indicator/controller is programmed to interpret a 4 mA signal as 0 psi and a 20 mA signal as 100 psi. Pressure relief valve: Final control element. In this example if the measured pressure is too high, the controller instructs the valve to open to reduce the pressure in the vessel. Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter
- 10. What can go wrong with a 4 to 20 mA loop? Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Wiring problems: Bad terminations, compromised insulation, corrosion and contami- nation can cause wiring to negatively impact 4 to 20 mA loop operation. 24V loop power supplies: Noisy, defective or overloaded power supplies can cause erratic mA loop operation or failures. Bad I/O into the controller: If the mA signal is correct and the controller does not interpret the mA signal correctly the control of the process is lost. Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter
- 11. What can go wrong with a 4 to 20 mA loop Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Bad transmitter: If the transmitter does not change the mA signal to correctly to respond to the measured PV the control system will not correct to adjust the PV correctly. Bad sensor or clogged capillary: If the temperature sensor is defective the transmitter cannot sense the temperature. In a pressure transmitter if the connection to the process is clogged the transmitter cannot measure the pressure accurately. Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter
- 12. How do you troubleshoot? What tools are of the most use? What can be measured or sourced Measurement and sourcing tool What it tells the technician Measure 4 mA to 20 mA signals DMM. Loop calibrator, mA clamp, ProcessMeter If the measured mA value is the expected value Source 4 mA to 20 mA signals Loop calibrator, mA clamp, ProcessMeter If the I/O or other mA input device is working correctly Simulate 4 mA to 20 mA signals Loop calibrator, Fluke 772 or 773 mA Clamp, ProcessMeter If the power supply, wiring and I/O is working correctly, perform transmitter substitution test Measure 24 V loop voltage DMM, Loop calibrator, Fluke 773 mA Clamp, ProcessMeter If the full 24V supply available, if it is defective or being loaded down? Supply 24 V loop voltage Loop calibrator, mA clamp, Fluke 789 ProcessMeter If a substitution test for installed supply fixes the problem Source 0 V to 10 V, 1 V to 5 V Loop calibrator with voltage source (715) or specialized mA Clamp (773) If the I/O or other voltage input device is working correctly Continuity measurements DMM, ProcessMeter, some multifunction process calibrators Find open circuits, bad terminations, resistive connections and mis-wires
- 13. Measure the 4 mA to 20 mA signal In series, “Break the loop” Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Compare measured mA signal to the expected value on the display
- 14. Measure the 4 mA to 20 mA signal; Don’t break the loop Don’t “Break the loop” Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Compare measured mA signal to the expected value on the display
- 15. Source a 4 mA to 20 mA signal Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Tests the indicator, controller or PLC/Control system I/O directly. Verify the value on the display
- 16. Simulate a 4 mA to 20 mA signal Simulate a transmitter in a loop, regulate current Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Tests the wiring, power supply, indicator, controller or PLC/Control system I/O. Verify the value on the display. Perform a transmitter substitution test.
- 17. Measure 24V loop power Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Is the full 24 V available or is the power supply being loaded down
- 18. Test with external 24 V loop power supply Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Pressure input Current output Percent of span 0 psi 4 mA 0 % 25 psi 8 mA 25 % 50 psi 12 mA 50 % 75 psi 16 mA 75 % 100 psi 20 mA 100 % Pressure transmitter Supply 24 V loop power, simultaneously measure mA signal. Substitution test for the power supply.
- 19. Using a HART smart transmitter as a mA source • Using loop test, the Fluke 744 issues the mA output command to the transmitter over the HART cable • Transmitter sources a mA signal into the I/O • Verify correct indication on the display • Compare commanded output value to mA signal read by Fluke 754 • Tests transmitter output, power supply, wiring and I/O HART cable Temperature input Current output Percent of span 0 °C 4 mA 0 % 75 °C 8 mA 25 % 150 °C 12 mA 50 % 225 °C 16 mA 75 % 300 °C 20 mA 100 % Temperature transmitter Instrument Output 4 to 20 mA output to PLC or DCS PVAO command (from communicator, Fluke 744 or 754) Loop test output trim Smart transmitter output
- 20. • The input to many control systems and PLCs is a voltage signal • 1 V to 5 V is most common as 4 to 20 mA through 250 ohms is 1 V to 5 V • Some low power transmitters have 1 V to 5 V outputs • Many older chart recorders and HVAC systems have 1 V to 5 V and 0 V to 10 V I/O 2200 ºC Readout / Controller DCS / PLC / Recorder Troubleshooting, sourcing 1 V to 5 V and 0 V to 10 V 0.004 x 250 = 1.0 V 0.020 x 250 = 5.0 V Ohms law
- 21. Testing I to P devices I to P converts a 4 mA to 20 mA signal to a 3 psi to 15 psi pneumatic signal •Often used with control valves •Used as a bridge between 4 mA to 20 mA loop and 3 psi to 15 psi pneumatic technology •Typically operate from a 20 psi or greater pressure supply •Source 4 mA to 20 mA, verify/measure correct pressure output 4-20 mA current input Pressure output Supply pressure ~20 PSI Current input Pressure output Percent of span 4 mA 3 psi 0 % 8 mA 6 psi 25 % 12 mA 9 psi 50 % 16 mA 12 psi 75 % 20 mA 15 psi 100 %
- 22. Testing control valves • Flow control, final control element • 4 mA to 20 mA or 3 psi to 15 psi input • Normally closed or open –Normally closed fails closed with loss of power –Normally open fails open with loss of power • Apply a 4 mA to 20 mA signal and check for operation per the applicable table below Normally Closed Normally Open Closed ClosedOpen Open Closed Open Pressure input Current output Percent of travel 3 psi 4 mA 0 % 6 psi 8 mA 25 % 9 psi 12 mA 50 % 12 psi 16 mA 75 % 15 psi 20 mA 100 % Pressure input Current output Percent of travel 3 psi 4 mA 0 % 6 psi 8 mA 25 % 9 psi 12 mA 50 % 12 psi 16 mA 75 % 15 psi 20 mA 100 %
- 23. Testing HART control valves • Testing HART valves require simultaneous HART feedback, while energizing the valve and monitoring travel. • Underperforming valves can disrupt a process unexpectedly, leading to expensive unplanned downtime. • Even valves with no visible issues, may be contributing to lowered quality and higher cost. • Simple tools exist for quick verification of valve health to avoid some common issues. • Often control valves are critical components in safety & shutdown systems What can go wrong with a Control Valve? Mechanical Issues: Electrical Issues: Equipment Issues: Stiction Hysteresis Dead Band Stick-Slip Over/Under Travel PV Translation Control Power Loss Out of Tolerance Positioner Communication Transmitter Malfunction Under/Over-Sized Valves Nonlinearity Applications
- 24. Review: Current loop devices and test methods • Transmitters –Apply input stimulus (temperature, pressure, etc) –Measure for correct 4 to 20 mA output –Use mA simulate for substitution testing • 24 V loop power supplies –Measure for correct voltage, substitution test to verify • I to P, 4 mA to 20 mA input, 3 psi to 15 psi output –Source 4 to 20 mA, verify/measure correct pressure output • Control valves –Source 4 mA to 20 mA, verify position indication –Use mA signal ramping to test for smooth operation • PLC, DCS, indicators, controllers, flow computers, and chart recorders analog inputs –Source 4 mA to 20 mA into the input and verify correct indication
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- 26. Questions or Comments? Email Nicole VanWert-Quinzi nvanwert@Transcat.com Transcat: 800-828-1470 www.Transcat.com For related product information, go to: www.transcat.com/brand/fluke-store